A pair of sloths have speeded up their usual slow dating game to produce London Zoo's first baby sloth - to the surprise of their keepers.

Zookeepers at ZSL London Zoo were shocked to discover two-toed sloth Marilyn was pregnant as they were unaware she had mated with male partner Leander, who arrived from Germany in 2012 to be paired with her.

Staff at the zoo had no idea the pair had acknowledged each other, but while they were playing it cool in public it seems that after hours the "sneaky" sloths had hit it off.

Two-toed sloths have been known to take nearly a decade to form a pair bond and breed, but factoring in an estimated 11-month pregnancy, it appears Marilyn and Leander were far from slow off the mark, taking six months to successfully mate.

Zookeeper Tegan McPhail said: "To say we were surprised when Marilyn fell pregnant is something of an understatement - we weren't aware that she'd even been near the male.

"When Leander arrived at the zoo at the end of 2012, he and Marilyn just didn't appear particularly interested in each other, but we knew that with sloths these things can take some time.

"However, with the arrival of the first sloth to be born at ZSL London Zoo, it appears that it was all an act on their behalf and our two very sneaky sloths were getting amorous behind our backs."

Keepers do not know the sex of the new arrival, which is spending its days clinging on to Marilyn, and will not be able to tell if it is a boy or a girl until it is scanned by vets as there are no external differences between males and females.

Sloths are slow-moving, tree-dwelling, nocturnal animals which feed on plants in their native tropical forests in Central and South America.

They have long, sharp claws, good for hanging from trees, but making walking laborious. Sloths are strong swimmers and can drop from trees into a river to swim across it.